The noncitizens registered to vote, in some cases not realizing what they were doing, Johnson said in a statement, "but the law is clear — only U.S. citizens may be registered to vote and vote on election day."
There are criminal penalties for noncitizens who register and vote, Johnson said.
"Voting is one of our most precious freedoms and is really the cornerstone of democracy," Johnson said. "I have an obligation to the citizens of this state to ensure that there is integrity in the system."
Department of state offices used to routinely ask all people getting driver's licenses if they wanted to register to vote but no longer do so, the department said.
The list of noncitizen voters comes from Michigan's driver records and federal records, Johnson said.
The department will send letters this week asking the individuals to contact state election officials to be removed from the rolls, Johnson said.
The department last year said it verified about 1,000 noncitizen registered voters and extrapolated that there could be as many as four times more.
The 600 represents those who could be verified as registered voters, department spokeswoman Gisgie Gendreau told The Associated Press. She said the federal government hasn't helped Michigan work through its voter registration list.
"For many years, our branches asked if customers wanted to register to vote," Gendreau said. She said it started a new policy in 2008 in which clerks were not supposed to ask people whose paperwork showed they weren't citizens about registering to vote.
In May 2013, the department reprogrammed its system so computers don't prompt staffers to ask people about registering to vote if their documents show they aren't U.S. citizens, she said.